“reCAPTCHA supplies subscribing websites with images of words that optical character recognition (OCR) software has been unable to read. The subscribing websites (whose purposes are generally unrelated to the book digitization project) present these images for humans to decipher as CAPTCHA words, as part of their normal validation procedures. They then return the results to the reCAPTCHA service, thereby contributing to the digitization project. The result is that the university receives approximately 3,000 man hours per day of free labor to help in the preservation of books.”